1. Field of the Invention
The present invention relates to a signal pickup object lens used for reading out signals from optical recording media such as CD, CD-ROM, and DVD and, in particular, to a lens for reading out an optical recording medium using a semiconductor laser as a light source.
2. Description of the Prior Art
In recent years, various kinds of optical disks such as CD and CD-ROM have made remarkable progress. As their read-out light source, there have been used semiconductor lasers which have such excellent characteristics as small size, light weight, and high efficiency and can be subjected to high-speed direct modulation.
While such a semiconductor laser has the advantages mentioned above, the aspect ratio of its active area is not 1:1, whereby its beam divergence angle (half angle) in the vertical direction differs from that in the horizontal direction with respect to the junction surface in general. Also, since the beam waist positions in these two directions differ from each other, an astigmatic difference is likely to occur. Accordingly, there may be cases where the beam emitted therefrom is not easily handled as it is.
Namely, in the case where such a light beam having an astigmatic difference is to be converged by means of a typical convex lens, even when the lens has been designed to converge the light beam to its diffraction limit, the converged light may have different waist positions in the above-mentioned two directions. Accordingly, it may become difficult to securely converge light onto a signal surface of the optical recording medium, whereby signals may not be easily read out with a high accuracy.